FFmpeg can be patched to enable 3gp conversions, useful for putting video onto mobiles. If the source name as amr in it then this has been done, otherwise use these patches:
wget http://www.3gpp.org/ftp/Specs/archive/26_series/26.073/26073-510.zip
wget http://www.3gpp.org/ftp/Specs/archive/26_series/26.104/26104-510.zip
wget http://www.3gpp.org/ftp/Specs/archive/26_series/26.204/26204-510.zip
26073-510.zip into libavcodec/amr
26104-510.zip into libavcodec/amr_float
26204-510.zip into libavcodec/amrwb_float
Install ffmpeg
>./configure --prefix=/usr/local --enable-shared --enable-pp --enable-swscaler --enable-pthreads --enable-x11grab --enable-a52 --enable-dts --enable-faac --enable-mp3lame --enable-libogg --enable-libtheora --enable-vorbis --enable-x264 --enable-xvid --enable-amr_nb --enable-gpl --cpu=pentium4
>make && make install && ldconfig
Unpack mplayer into /usr/local/src
Unpack ffmpeg and copy the libavcodec, libavutil and libavformat subdirectories into /usr/local/src/mplayer
Install mplayer with:
>CFLAGS= ./configure --prefix=/usr/local --enable-gui --with-extralibdir=/usr/lib/win32
>make
>make install && ldconfig
>ln -s /usr/share/fonts/DejaVuSans.ttf /home/username/.mplayer/subfont.ttf
Unpack the font package to /usr/local/share/mplayer/font
Unpack the skins packages into /usr/local/share/mplayer/skins